For phase contrast imaging based on slit-scanning, for example for differential phase contrast imaging (DPCI), a required pre-processing step is flat field correction in the imaging applications. DPCI is an emerging technology that has the potential to improve the diagnostic value of X-ray imaging. A DPCI system may be provided with three gratings used between the X-ray source and the detector. The acquisitions of several X-ray images at different relative positions of two of the gratings close to the detector are required. Since these gratings have pitches in the order of a few micrometers only, there are tight requirements on the accuracy of the stepping device that performs the relative movement of the gratings. For example, U.S. 2012/0099702 A1 describes a correction method for differential phase contrast imaging. Furthermore, U.S. 2011/243305 A1 discloses a system for X-ray phase contrast imaging as well as a phantom for sensitivity correction of such system. In addition, WO 2013/004574 A1 discloses a slit scanning system for generating X-ray phase contrast images. Further, for a large object, a large field of view is necessary in case the object has to be imaged by a single phase stepping series. Alternatively, systems may be used with a small field of view, but in addition a scan of the object relative to the imaging system may be provided. Either the imaging system is moved relative to the object, or the object is moved with respect to a fixed imaging system. A prerequisite is that a phase shift of at least one interference fringe period of the interferometer occurs. However, the fringe profile achieved may not look like the ideal fringe pattern. It has been shown that due to the internal electrical gains, as well as due to individual pixel surface areas, the individual detective quantum efficiencies and the individual X-ray transmission factors of the system, the detected fringe distribution shows up being systematically disturbed.